In today's terminology, a “hybrid vehicle” is a vehicle that uses two or more distinct power sources to propel the vehicle. Most of today's hybrid vehicles use an on-board rechargeable energy storage system and a fueled power source (internal combustion engine or fuel cell). The term “hybrid vehicle” most commonly refers to a hybrid-electric vehicle (HEV), which has both an electric motor/battery system and an internal combustion engine.
One type of modern mass-produced HEV prolongs the charge on the motor battery by capturing kinetic energy via regenerative braking. Other HEV's use the engine to generate electricity by spinning an electrical generator (often a motor-generator), which either recharges the battery or directly delivers power to a motor.
HEV's can be categorized as being either parallel or series systems. Parallel systems have both an internal combustion engine and an electric motor connected to a mechanical transmission. Most designs combine a large electrical generator and a motor into one unit, often located between the combustion engine and the transmission, replacing both the conventional starter motor and the alternator. The two sources of power (engine and motor) are mechanically coupled with a differential gear.
Series hybrid vehicles tend to be more similar in design to a battery electric vehicle than an internal combustion vehicle or parallel hybrid. In a series system, the combustion engine drives an electric generator instead of directly driving the wheels. The generator both charges a battery and powers an electric motor that moves the vehicle. When large amounts of power are required, the motor draws electricity from both the battery and the generator. Thus, a feature of a series hybrid is the lack of a mechanical link between the combustion engine and the wheels.
Combined hybrid systems have features of both series and parallel hybrids. They incorporate power-split devices allowing for power paths from the engine to the wheels that can be either mechanical or electrical. The main principle behind such a system is the decoupling of power supplied by the engine from power demanded by the driver.